Polyamide powder for selective sintering methods

ABSTRACT

A polyamide powder for selective absorbing sintering, SAS, or selective inhibition sintering, SIS. The polyamide powder has a solution viscosity to ISO 307 of 1.8 to 2 and a rise in the solution viscosity of 0% to 25% when it is subjected to a temperature 20° C. below its melting temperature under air for 20 hours.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of German Patent ApplicationNo. 102016219080.6, filed Sep. 30, 2016, the contents of which areincorporate herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to polyamide powders for use in selectiveabsorbing sintering, SAS, or selective inhibition sintering, SIS, and tothe use thereof. The invention further relates to shaped bodies and tothe production thereof.

Description of the Related Art

Additive manufacturing methods, frequently also referred to as rapidprototyping, are used in order to be able to quickly and inexpensivelymanufacture three-dimensional objects. This manufacturing is effecteddirectly on the basis of the in-computer data models from shapeless(liquids, powders or the like) or shape-neutral (in ribbon or wire form)material by means of chemical and/or physical processes. Polymer powdersin particular, such as polyamide powder, are suitable as shapelessmaterial.

Powder bed fusion methodology includes, among other techniques, directmetal laser sintering (DMLS), electron beam melting (EBM), selectiveheat sintering (SHS), selective laser melting (SLM), selective lasersintering (SLS), selective absorbing sintering (SAS) and selectiveinhibition sintering (SIS). The SAS method is described, for example, inUS 2007/238056. US 2004/137228 A1 presents the SIS method.

The selectivity of the laser-free processes can be accomplished, forexample, via absorbers (selective absorbing sintering, SAS) orinhibitors (selective inhibition sintering, SIS). In the SAS method, theabsorption of the material in contact with the absorber is increased; bycontrast, the inhibitor retards the melting. Absorber and inhibitor canbe used together in one method. Suitable energy sources in SAS methodsare those that are incorporated into the material only to a limiteddegree. In the SIS method, the energy source should be chosen such thatthe material is heated sufficiently rapidly.

Absorbers and inhibitors can be applied to the material in dissolved ordispersed form in a liquid, for example by means of inkjet methods inthe form of inks. The liquid or absorber and inhibitor should merely beabsorbed by the material printed and not flow away horizontally orvertically within the material.

As a result of the use of the polyamide powder in molding production,which typically takes place 10 to 20 K below the melting temperature,aging phenomena can occur. In this context, the amine and carboxylicacid end groups react with one another and cause extension of thepolyamide chains. Reprocessing of the powder is no longer possible, andso the unprocessed powder has to be exchanged.

SUMMARY OF THE INVENTION

The problem addressed was thus that of providing a polyamide powderwhich can be used in SAS or SIS processes, wherein the unprocessedpolyamide powder can be employed again. This can reduce costs andprotect the environment. The moldings obtained, in spite of the use ofpolyamide powder from previous molding productions, should exhibithomogeneous properties. Moreover, the moldings that are obtained by SASor SIS, compared to selective laser sintering SLS, should havedistinctly improved mechanical properties such as elevated elongation atbreak.

Accordingly, polyamide powders for selective absorbing sintering, SAS,or selective inhibition sintering, SIS, have been found, which do nothave the disadvantages of the prior art. The polyamides have a solutionviscosity to ISO 307 of 1.8 to 2. In addition, the rise in the solutionviscosity is 0% to 25%, preferably 5% to 15%, when the polyamide powderis subjected to a temperature 20° C. below its melting temperature underair for 20 h.

A temperature of 20° C. below the melting temperature under air for 20 hsimulates conditions that exist in a construction space for productionof moldings.

DETAILED DESCRIPTION OF THE INVENTION

The problem was solved by a polyamide powder having a relatively highinitial viscosity and only a slight rise in the solution viscosity overthe period of 20 h. As a result, it can be reused repeatedly.

Solution viscosity is determined in a double determination according toISO 307 using the following parameters: Schott AVS Pro, solvent: acidicm-cresol, volumetric method, dissolution temperature 100° C.,dissolution time 2 h, polymer concentration 5 g/l, measurementtemperature 25° C.

To determine the rise in the solution viscosity, the powder is subjectedto a temperature 20° C. below its melting temperature under air for 20h. The solution viscosity of the respective powders is subsequentlydetermined as specified above.

The melting temperature is determined by means of differential scanningcalorimetry (DSC) to DIN 53765. The crucial parameter is the meltingtemperature in the first heating step. The heating and cooling rates areeach 20 K/min. The measurements are effected by means of a DSC 7 fromPerkin Elmer.

Preferably, the polyamide has either amine end groups in excess orcarboxylic acid end groups in excess. The excess can be achieved bymeans of diamines or dicarboxylic acids, preferably dicarboxylic acids.Based on the mass of the polyamide powder, the excess of one end groupover the other end group is 20 to 60 mmol/kg.

In a preferred embodiment of the invention, the polyamide powder hasopen mesopores, and the cumulative pore volume distribution of themesopores, measured to DIN 66134, is at least 0.01 cm³/g. The cumulativepore volume distribution is more preferably at least 0.025 cm³/g andmost preferably at least 0.035 cm³/g. Further-preferred cumulative porevolume distributions are respectively at least 0.045 cm³/g, 0.05 cm³/g,0.06 cm³/g and 0.07 cm³/g. Preferably, the cumulative pore volumedistribution is not more than 0.15 cm³/g and more preferably not morethan 0.1 cm³/g. In a further preferred embodiment of the invention, thecumulative pore volume distribution is 0.05 cm³/g to 0.15 cm³/g, morepreferably 0.06 cm³/g to 0.1 cm³/g.

Through the open pores, absorber or inhibitor moves from the surfaceinto the particle interior and can thus enable more homogeneousdistribution of these substances. This results in more homogeneousmelting. In addition, horizontal or vertical running of the liquid isprevented, since the liquid is absorbed in a relatively high amount bythe particles—by contrast with superficial absorption.

The pores bring about, through capillary forces, absorption of absorberor inhibitor in the form of the liquid. The effect of the cumulativepore volume distribution of at least 0.01 cm³/g is that absorber orinhibitor is absorbed very quickly; ultimately, during the SAS or SISmethod, it is customary for temperatures to exceed 100° C., which leadsto rapid evaporation of the liquid. In this case, absorbers orinhibitors should not run or merge into one another. If the cumulativepore volume distribution were to be below at least 0.01 cm³/g, theliquid would evaporate before absorbers or inhibitors had penetratedinto the particles. Absorbers would thus remain on the surface of theparticles as in the prior art, and the inhibitor would achieve lowinhibitor performance.

Open pores of the particles are connected to the surrounding medium,whereas closed pores are intrinsically closed off and do not allow anymedium to penetrate. Fine pores having a diameter of 20 μm or less aresubdivided by IUPAC into macropores (>50 mn), mesopores (2-50 nm) andmicropores (<2 nm). A preferred polyamide powder has at least 30%, morepreferably at least 50%, open mesopores, based in each case on the sumtotal of open macro- and mesopores of the polyamide powder having a porediameter of 2 to 300 nm, measured to DIN 66134. The standard applies tomesoporous solids, but the range above 50 nm was likewise determinedaccording to this standard.

Polyamides having micropores are less suitable, since they cannot absorbthe liquid quickly enough and cannot absorb customary absorbers at all.Macropores can exhibit reduced capillary action and likewise do not leadto sufficiently rapid absorption of the liquid containing absorber orinhibitor into the particle interior.

The polyamide powder preferably absorbs 1,000 pl to 30,000 pl of liquidper gram of polyamide powder, preferably 3,000 pl to 25,000 pl, and morepreferably 5,000 pl to 20,000 pl.

Suitable liquid is typically any compressible liquid which containsabsorber or functions as inhibitor. The liquid in which the absorber orthe inhibitor is dissolved or dispersed is preferably selected from thesolvents water, monoalcohols having 1 to 4 carbon atoms, glycol, ormixtures thereof.

In one embodiment of the invention, the polyamide powder includes eitherabsorbers or inhibitors. For this purpose, the polyamide powder has beencontacted with the liquid described above. This can be effected, forexample, by known printing methods.

The absorbers or inhibitors may be colorants. As used herein, “colorant”is the umbrella term for all coloring substances. They can be dividedinto dyes and pigments by their solubility in the surrounding mediumaccording to DIN 55944:1990-04. Dyes are organic black or coloredsubstances that are soluble in the surrounding medium. Pigments, bycontrast, are colorants in the form of powders or platelets that, bycontrast with dyes, are insoluble in the surrounding medium. Theparticle size is typically 30 to 200 nm (laser diffraction). Thecolorant is preferably a pigment. Preferably, the pigment is selectedfrom the group consisting of organic and inorganic, color, effect, colorand effect, magnetically shielding, electrically conductive,corrosion-inhibiting, fluorescent and phosphorescent pigments.Preference is given to using the color and/or effect pigments.

Suitable pigments are selected from chalk, ochre, umber, green earth,burnt sienna, graphite, titanium white (titanium dioxide), lead white,zinc white, lithopone, antimony white, carbon black, iron oxide black,manganese black, cobalt black, antimony black, lead chromate, lead oxidered, zinc yellow, zinc green, cadmium red, cobalt blue, Prussian blue,ultramarine, manganese violet, cadmium yellow, Paris green, molybdateorange, molybdate red, chromium orange, chromium red, iron oxide red,chromium oxide green, strontium yellow, metal effect pigments,pearlescent pigments, luminous pigments comprising fluorescent andlorphosphorescent pigments, gamboge, bone charcoal, Kassel brown, indigo,chlorophyll, azo dyes, indigoids, dioxazine pigments, quinacridonepigments, phthalocyanine pigments, isoindolinone pigments, perylenepigments, perinone pigments, metal complex pigments, alkali bluepigments and diketopyrrolopyrrole.

In order to achieve better processability of the polyamide powder, itmay be advantageous that additives are added. Additives of this kindmay, for example, be free-flow aids. More preferably, the polyamidepowder includes 0.05% to 5% by weight, preferably from 0.1% to 1% byweight, based on the total weight of the polyamide powder, of additives.Free-flow aids may, for example, be fumed silicas, stearates or otherfree-flow aids known from the literature, for example tricalciumphosphate, calcium silicates, Al₂O₃, MgO, MgCO₃ or ZnO. Fumed silica issupplied, for example, under the AEROSIL brand name (Evonik IndustriesAG).

As well as or instead of such free-flow aids, some of which areinorganic, or other additives, the polyamide powder may also includeinorganic filling materials. The use of such filling materials has theadvantage that these essentially retain their shape through thetreatment in the bonding operation and hence reduce shrinkage of theshape body. Moreover, it is possible through the use of fillingmaterials, for example, to alter the plastic and physical properties ofthe objects. Thus, through use of powder material including metalpowder, both the transparency and color and the magnetic or electricalproperties of the object can be adjusted. As fillers or fillingmaterials, the powder material may include, for example, glassparticles, ceramic particles or metal particles. Typical fillers are,for example, metal granules, aluminum powder, steel shot or glass beads.Particular preference is given to using powder materials including glassbeads as filling materials. In a preferred embodiment, the powdermaterial according to the invention includes from 1% to 70% by weight,preferably from 5% to 50% by weight, and most preferably from 10% to 40%by weight of filling materials, based on the total weight of thepolyamide powder.

The surface energy of the liquid should be less than the surface energyof the polyamide powder.

In embodiments, the liquid contains the colorant preferably in aproportion of 0.1% to 10% by weight, more preferably 2.5% to 5% byweight, based on the total weight of the liquid. The pH of the liquid istypically adjusted to the range of 6 to 9.

Suitable liquids may be commercial inks which are supplied for inkjetprinting.

Suitable polyamides for the polyamide powder may be customary and knownpolyamides. Polyamides include homopolyamides and copolyamides. Suitablepolyamides or copolyamides are selected from nylon-6, -11, -12, -10,13,-10,12, -6,6, -4,6, -6,13, -10,6, -11/10,10, -12,12 and -12/10,12. Apreferred polyamide is selected from nylon-11, -12, -10,13, -10,12,-11/10,10, -12,12 and -12/10,12, more preferably nylon-11 or -12, andmost preferably nylon-12.

Typically, a polyamide powder which is used in sintering methods shouldhave a minimum BET surface area. The prior art discloses that the valueshould be, for example, less than 7 m²/g. The polyamide powder accordingto the invention, by contrast, should preferably have a BET surfacearea, measured to DIN ISO 9277, of at least 7 m²/g, preferably in arange of from 7.5 m²/g to 30 m²/g. A particularly preferred embodimentincludes polyamides having a BET surface area of at least 7 m²/g,preferably of 7.5 m²/g to 30 m²/g. Measurement is effected with theMicromeritics TriStar 3000 instrument, nitrogen gas adsorption,discontinuous volumetric method, 7 data points at relative pressuresP/PO from about 0.05 to about 0.20, dead volume calibration using He(99.996%), sample preparation 1 h at 23° C.+16 h at 80° C. in vacuo,specific surface area based on devolatilized specimen, evaluation bymeans of multipoint determination.

In a preferred embodiment, the polyamide powder has a cumulative porevolume distribution of at least 0.02 cm³/g and a BET surface area of atleast 2.8 m²/g, preferably 0.04 cm³/g to 5.8 m²/g, more preferably 0.05cm³/g to 10 m²/g and especially preferably of 0.07 cm³/g to 13 m²/g.

The weight-average particle diameter d₅₀ of the polyamide powder,measured by means of laser diffraction, should be preferably not morethan 100 gm, preferably 10 gm to 80 gm (Malvern Mastersizer 3000; wetdispersion was effected in water, refractive index and blue light valuefixed at 1.52; evaluation via Mie theory; dry measurement, 20-40 g ofpowder metered in by means of a Scirocco dry disperser; vibratingchannel feed rate 70%, dispersion air pressure 3 bar; measurement timefor the sample 5 seconds (5000 individual measurements)). Polymershaving such diameters are also referred to as polymer powder.

It is advantageous when the polyamide powder with a particle diameter ofless than 10.48 μm (ultrafine particles) is present in a small amount.The proportion of ultrafine particles should be less than 3% by weight,preferably less than 1.5% by weight and more preferably less than 0.75%by weight, based in each case on the total weight of polyamide powder.This reduces the evolution of dust and enables an improvement inprocessability. Ultrafine particles can be removed, for example, bymeans of sifting.

Preference is further given to polyamide powders having a bulk density,measured to DIN 53466, between 300 g/I and 600 g/l.

In addition, polyamides having a surface energy of not more than 35mN/m, preferably from 25 mN/m to 32 mN/m, are preferred polyamides. Thesurface energy is determined by means of contact angle measurement bythe capillary rise height method by use of the Washburn equation and theevaluation method according to Owens, Wendt, Rabel and Kaelble.Polyamide powders of this kind have very homogeneous flowability, whichresults in a high dimensional stability of the shaped bodies.

The polyamide powder and the composition thereof can be obtained bygrinding the powder produced or by a precipitation process(re-precipitation). The polyamide powders obtained by the preferredprecipitation process typically feature a higher cumulative pore volumedistribution than polyamides which are obtained by grinding methods, forexample. Polyamides which are not obtained by a precipitation processbut by grinding methods in particular have a cumulative pore volumedistribution of generally well below 0.01 cm-³/g.

In the precipitation process, the polyamide is at least partly dissolvedat elevated temperature and then precipitated by reducing thetemperature. Suitable solvents for polyamides are, for example, alcoholssuch as ethanol. US5932687 mentions suitable process conditions, forexample. To establish the desired property, it is advantageous to leavethe suspension obtained at a temperature 2-4 K above the precipitationtemperature for 10 min to 180 min after the precipitation.

The invention further provides a process for production of theaforementioned polyamide powder. In embodiments, the production processcomprises polymerization and/or polycondensation of monomers to give apolyamide (step a) and powder production by grinding or re-precipitation(step b). In step a, either diamines are added as chain transfer agentto achieve an amine end group excess, or dicarboxylic acids are added toachieve a carboxylic acid end group excess. The diamines or dicarboxylicacids are preferably added in such a ratio that an excess of one of theend groups is 20 to 60 mmol/kg over the other end group (based on themass of the polyamide powder).

Suitable monomers are, for example, monomers suitable for production ofthe polyamides nylon-6, -11. -12, -10,13, -10,12. -6,6, -4,6, -6,13,-10,6. -11/10,10, -12,12 and -12/10,12.

Suitable diamines and dicarboxylic acids for establishment of the excessof end groups may be the same as or different from the monomers of thepolyamides. Examples include tetramethylenediamine,hexamethylenediamine, decanediamine, dodecanediamine, adipic acid,sebacic acid, dodecanoic acid, brassylic acid. It is preferable that thediamines or dicarboxylic acids have the same number of carbon atoms asthe monomers of the polyamides.

In one embodiment of the invention, the polyamide can be obtained byco-precipitation. For this purpose, in step a, at least one polyamide ofthe AB type, prepared by polymerization of lactams having 4 to 14 carbonatoms in the monomer unit or by polycondensation of the correspondingw-aminocarboxylic acids having 4 to 14 carbon atoms in the monomer unitand at least one polyamide of the AABB type, prepared bypolycondensation of diamines and dicarboxylic acids each having 4 to 14carbon atoms in the monomer units, is obtained. In this case, the powderis obtained in step b by co-precipitation of the at least one polyamideof the AB type and the at least one polyamide of the AABB type.

The invention further provides for the use of the polyamide powderaccording to the invention in SAS or SIS methods for production ofshaped bodies. It is preferable here to contact the polyamide powderwith the liquid.

In addition, shaped bodies which are obtained at least partly frompolyamide powders according to the invention form a further part of thesubject-matter of the invention. Furthermore, processes for producingshaped bodies by means of SAS or SIS methods, wherein the polyamidepowder according to the invention is used, likewise form part of thesubject-matter of the invention.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting unless otherwise specified.

EXAMPLE

A nylon-12 was produced. As well as laurolactam as monomer, dodecanoicacid was used in order to obtain an excess of dicarboxylic acid endgroups. The powder was obtained by means of a precipitation process.

The melting temperature and the solution viscosity of the obtainedpowder were determined. Thereafter, the powder was subjected to atemperature 20° C. below its melting temperature under air for 20 hours,and the solution viscosity was determined constantly.

Melting temperature: 185° C.

Temperature for simulation of ageing: 165° C.

Time/h Solution viscosity 0 1.88 1 1.99 2 2.02 4 2.05 8 2.08 20 2.07

Numerous modifications and variations on the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A polyamide powder for selective absorbing sintering, SAS, orselective inhibition sintering, SIS, wherein the polyamide has asolution viscosity to ISO 307 of 1.8 to 2 and a rise in the solutionviscosity of 0% to 25%, when subjected to a temperature 20° C. below itsmelting temperature under air for 20 hours.
 2. The polyamide powderaccording to claim 1, wherein the polyamide has a rise in solutionviscosity of 5 to 15% when subjected to a temperature 20° C. below itsmelting temperature under air for 20 hours.
 3. The polyamide powderaccording to claim 1, wherein the polyamide comprises either amine endgroups in excess or carboxylic end groups in excess.
 4. polyamide powderaccording to claim 3, wherein the excess is achieved by diamines ordicarboxylic acids.
 5. The polyamide powder according to claim 3,wherein the excess is achieved by dicarboxylic acids.
 6. The polyamidepowder according to claim 3, wherein the excess is 20 to 60 mmol/kg ofpolyamide powder.
 7. The polyamide powder according claim 1, wherein thepowder has open mesopores and the cumulative pore volume distribution ofthe mesopores, measured to DIN 66134, is at least 0.01 cm³/g.
 8. Thepolyamide powder according to claim 1, wherein the polyamide powderabsorbs liquid in a volume of 1,000 pl/g to 30,000 pl/g.
 9. Thepolyamide powder according to claim 1, wherein a BET surface area of thepolyamide powder, measured to DIN ISO 9277, is at least 7 m²/g.
 10. Thepolyamide powder according to claim 1, wherein a BET surface area of thepolyamide powder, measured to DIN ISO 9277, is from 10 m²/g to 30 m²/g.11. The polyamide powder according to claim 1, Wherein a weight-averageparticle diameter d₅₀ of the polyamide powder, measured by laserdiffraction, is not more than 100 μm.
 12. The polyamide powder accordingto claim 1, wherein a weight-average particle diameter d₅₀ of thepolyamide powder, measured by laser diffraction, is from 10 μm to 80 μm.13. The polyamide powder according to claim 1, wherein a bulk density ofthe polyamide powder, measured to DIN 53466, is between 300 and 600 g/l.14. The polyamide powder according to claim 1, wherein a surface energyof the polyamide, determined by a contact angle measurement by acapillary rise height method by use of the Washburn equation and theevaluation method according to Owens, Wendt, Rabel and Kaelble, is notmore than 35 mN/m.
 15. The polyamide powder according to claim 1,wherein a surface energy of the polyamide, determined by a contact anglemeasurement by a capillary rise height method by use of the Washburnequation and the evaluation method according to Owens, Wendt, Rabel andKaelble, is from 25 mN/m to 32 mN/m.
 16. The polyamide powder accordingto claim 1, wherein the polyamide powder is obtained by a precipitationprocess.
 17. A process for producing the polyamide powder according toclaim 1, comprising: (a) polymerizing and/or polycondensing monomers togive a polyanaide; and (b) producing a powder by grinding orprecipitation, wherein diamines or dicarboxylic acids are added as achain transfer agent in (a) to achieve an amine end group excess or acarboxylic acid end group excess, respectively.
 18. The processaccording to claim 12, wherein the diamines or dicarboxylic acids areadded in such a ratio as to achieve an excess of 20 to 60 mmol/, basedon a mass of the polyamide powder.
 19. Shaped bodies which are obtainedat least partly from the polyamide powder according to claim
 1. 20. Aprocess for producing shaped bodies, comprising using the polyamidepowder according to claim 1 in a SAS method or a SIS method.